This invention pertains to rotary disc reactors in which a stack of semiconductor wafers is subjected to chemical vapor deposition of material for epitaxial growth on the wafers. More particularly, this invention pertains to oscillation of a rotatable gas manifold which is used in such rotary disc reactors to sweep a stream of gas across the semiconductor wafers in order to equalize epitaxial growth across the surfaces of the semiconductor wafers.
U.S. Pat. No. 4,062,318 to Ban et al. discloses apparatus for use in chemical vapor deposition (CVD) in a rotary disc reactor. As is there set forth, it has been found that if the gas manifold and the semiconductor wafers are held fixed with respect to each other, the epitaxial layer which is grown on the wafer substrate will be non-uniform in thickness and will be thickest at the central region of the wafer. This patent teaches that more uniform growth of an epitaxial layer may be achieved by oscillating the gas manifold back and forth to sweep a stream of gas across the surface of each semiconductor wafer. This patent also teaches that it is advantageous to increase sweep speed when the stream of gas is swept across the central regions of the semiconductor wafers and to decrease sweep speed when the gas is swept across the peripheral regions of the semiconductor wafers. Since the central regions of the semiconductor wafers are always swept for less time than are the peripheral regions of the semiconductor wafers, excessive growth rates of the epitaxial layer at the central regions of the semiconductor wafers are reduced if not eliminated.
In order to achieve such time-controlled sweep, an electric motor has been connected directly to the gas manifold, the electric motor being controlled by a programmable instrument. Such apparatus has been found to be expensive, bulky, and difficult to program. There is a need for a compact, inexpensive and more easily programmed system for oscillating a gas manifold at a predetermined sweep rate in a rotary disc reactor.